The U.S. has proven reserves of natural gas totaling over 150 trillion cubic feet. Recently, annual consumption has exceeded the amount of new reserves that were found. This trend has resulted in higher cost natural gas and may possibly result in supply shortages in the future. As the U.S. reserves are produced and depleted, finding new, clean gas reserves involves more costly exploration efforts. This usually involves off-shore exploration, deeper drilling onshore and/or the production of low volume “unconventional” wells all of which are expensive. Moreover, unlike crude oil, it is expensive to liquefy natural gas so that the liquid can be shipped or otherwise transported from areas of production or excess supply and revaporized for local use. Therefore, pricing of natural gas can be expected to rise forcing end users to seek alternative fuels, such as oil and coal, that are not as clean burning as gas. While base consumption for natural gas in the U.S. is projected to grow at 2-3% annually for the next ten years, one segment may grow much more rapidly. Natural gas usage in electric power generation is expected to grow rapidly because natural gas is efficient and cleaner burning allowing utilities to reduce atmospheric emissions. Further use of natural gas as a transportation fuel is a rapidly growing market due to its clean burning features and low cost relative to liquid fuels. Accordingly, there is a need to develop a cost-effective method of upgrading currently unmarketable sub-quality natural gas reserves in the U.S. thereby increasing the proven natural gas reserve inventory.
In addition to the benefits of using landfill gas and other sub-quality natural gas reserves as a source of industrial fuel to meet demand, methane emissions from these various sources also represent a significant portion of non-CO2 greenhouse gas emissions. Traditionally, coal mine and landfill operators have been able to recover only a small percentage of dilute methane gas streams because they contain significant quantities of contaminants, such as CO2, oxygen, water vapor, and nitrogen. Removal of methane from sub-quality sources has the potential to reduce annual greenhouse gas emissions by about 23.5 billion equivalent kilograms of CO2 and to enable to cost-effective recovery of about 3.5 trillion cubic feet per year of natural gas. This represents a reduction of about 0.3% in annual U.S. greenhouse gas emissions at no net cost when the value of pipeline quality natural gas is realized.
When garbage is collected in a sanitary landfill, the decay of the contents leads to the generation of various gases, predominantly methane and carbon dioxide. Landfill gas can also contain nitrogen and oxygen, which is commonly introduced because the landfill gas is collected at low pressure and pulling on the gathering system used to collect the gas can introduce air through various leaks. Upgrading the methane gas from landfills has been widely practiced, most commonly for the production of electric power, but also to produce a high quality synthetic natural gas. Landfill gas compositions include a feed stream that is typically 50% methane, 40% CO2, and with the balance composed of primarily nitrogen plus oxygen with a typical oxygen level of 1%.
One of the major concerns with upgrading landfill gas and other sources of sub-quality natural gas reserves, both for electric power generation or for various fuel consumers, including pipeline gas, is that landfill gas contains a wide variety of impurities. One impurity of concern in landfill gas and other sources of natural gas is oxygen. Oxygen is present in landfill gas and coalmine methane in almost all cases and is occasionally present in other methane and natural gas streams. Pipeline acceptance of gases from such sources requires that the gas meet pipeline quality standards, which include specific limits on impurities such as water vapor, hydrogen sulfide, carbon dioxide, nitrogen and oxygen. The removal of these impurities, with the exception of oxygen, is well-proven and well-established. Further, the pipeline specifications on permitted levels of oxygen range widely and there are cases requiring a low of 10 ppm of oxygen to a high of 1% oxygen or more. This large range of permitted oxygen impacts the technologies and processing for its removal.
Currently, a bulk of the oxygen can be removed from a landfill gas feed during the removal of the CO2 impurity. Upgrading landfill gas by removing the CO2 impurity encompasses a variety of technologies, including solvent-based CO2 wash systems, membrane units where the CO2 is removed by permeation from high pressure to low pressure, and PSA systems where the CO2 is adsorbed and removed from the landfill gas. Of these technologies, the membrane unit offers the added advantage that oxygen will permeate the membrane to a substantial extent when the CO2 is removed. Using a membrane to upgrade the landfill gas permits the removal of the oxygen to moderate levels. Generating a product stream of 2,000 to 5,000 ppm of oxygen can be easily accommodated. When pipeline requirements call for moderate levels of oxygen (i.e., oxygen at 2,000 to 5,000 ppm), landfill gas upgrading by compression, pretreatment for heavy components removal and a membrane unit for the removal of the bulk of the CO2 and oxygen is the preferred approach. This technology has been applied at over a dozen landfills to date.
The use of membranes for gas separations, which is well-known in the art, is done by contacting the feed stream with the surface of the membrane at an elevated pressure and withdrawing the permeate stream at a reduced pressure, relative to the elevated feed pressure. Significant factors in the design and overall efficiency of membrane systems are the total membrane surface area required for a given separation and the partial pressure difference across the membrane that is required to obtain a desired product quantity and quality. The design of membrane systems requires a balancing of these factors. That is, the greater the partial pressure difference, or driving force, across the membrane, the less is the membrane surface area required for a given separation. High pressure difference, low area operation necessitates the use of more expensive compression equipment and higher compressor operating costs, but enables membrane equipment costs to be kept relatively low. If, on the other hand, a lower driving force is employed, more membrane surface area is required, and the relative costs of the various aspects of the overall system and operation would change accordingly.
Membrane systems are often designed and optimized for full capacity, steady flow and composition conditions that are not always encountered in practice. When conditions exist that are different than the design conditions, the products recovered from the membrane system may contain undesirable concentrations of certain components. Under such conditions, different requirements exist with respect to partial pressure differences and membrane area in order to maintain a given product purity. In addition, multiple stage membrane units with permeate recompression and additional stage treatment or recycle to the feed are widely practiced to optimize the product purity and/or recovery of the system.
However, where the product oxygen permitted into the pipeline is lower than about 2,000 ppm, and especially less than 1000 ppm, the use of the membrane unit for oxygen removal results in substantially lower methane recovery. Increasing the amount of oxygen removed using the membrane unit also entails increasing the amount of CO2 removed. This results in additional methane losses where additional methane is permeated across the membrane.
In order to meet the stricter pipeline requirements, a catalytic system can be considered for oxygen removal after the membrane removal step. In a catalytic system, oxygen is reacted over a catalyst bed wherein the methane in the landfill gas (or injected heavier hydrocarbons or other reactive species) and the oxygen form water and CO2. The amount of water and CO2 formed is related to the concentration of oxygen in the feed. Such catalytic systems are well-known in industry and have been applied at facilities where oxygen is required to be removed to typically low ppm levels, such as 10 ppm. However, using a catalytic system to remove oxygen from the landfill gas stream is expensive and results in loss of methane value since the methane is converted to water and carbon dioxide. Thus, an alternative to removing oxygen by using a catalytic system is needed.
Pressure swing adsorption is a well-known method for the separation of bulk gas mixtures and for the purification of gas streams containing undesirable impurities. Gas separations by pressure swing adsorption (PSA) are achieved by coordinated pressure cycling of a bed of adsorbent material which preferentially adsorbs at least one more readily adsorbable component present in a feed gas mixture relative to at least one less readily adsorbable component present in the feed gas mixture. That is, the bed of adsorbent material is contacted with a ready supply of a feed gas mixture. During intervals while the bed of adsorbent material is subjected to the ready supply of feed gas mixture and the bed is at or above a given feed pressure, a supply of gas depleted in the at least one more readily adsorbable component may be withdrawn from the bed. Eventually, the adsorbent material in the bed becomes saturated with the at least one more readily adsorbable component and must be regenerated. At which point, the bed is isolated from the ready supply of feed gas mixture and a gas enriched in the at least one more readily adsorbable component is withdrawn from the bed, regenerating the adsorbent material. In some instances, the bed may be subjected to a purge with depleted gas to facilitate the regeneration process. Once the adsorbent material is sufficiently regenerated, the bed is again subjected to the ready supply of feed gas mixture and depleted gas can once again be withdrawn from the bed once the pressure on the bed is at or above the given feed pressure. This cycle may be performed repeatedly as required.
The use of PSA systems for the removal of impurities, such as nitrogen and carbon dioxide, from natural gas streams are well known and used in the purification of natural gas streams. In general, an effective PSA process for the removal of nitrogen from natural gas, described in U.S. Pat. No. 6,197,092, issued Mar. 6, 2001, involves a first pressure swing adsorption of the natural gas stream to selectively remove nitrogen and produce a highly concentrated methane product stream. Secondly, the waste gas from the first PSA unit is passed through a PSA process which contains an adsorbent selective for methane so as to produce a highly concentrated nitrogen product. One important feature is the nitrogen selective adsorbent in the first PSA unit. The adsorbent is a crystalline titanium silicate molecular sieve adsorbent and is based on ETS-4, which is described in U.S. Pat. No. 4,938,939. Adsorbents having controlled pore sizes are referred to as CTS-1 (contracted titano silicate-1) and are described in U.S. Pat. No. 6,068,682, issued May 30, 2000. The CTS-1 molecular sieve is particularly effective in separating nitrogen and acid gases selectively from methane. Due to the ability of the ETS-4 compositions, including the CTS-1 molecular sieves for separating gases based on molecular size, these molecular sieves have been referred to as Molecular Gate® sieves.
There are also numerous patents that describe PSA processes for separating carbon dioxide from methane or other gases. One of the earlier patents in this area is U.S. Pat. No. 3,751,878, which describes a PSA system using a zeolite molecular sieve that selectively adsorbs CO2 from a low quality natural gas stream operating at a pressure of 1000 psia, and a temperature of 300° F. The system uses carbon dioxide as a purge to remove some adsorbed methane from the zeolite and to purge methane from the void space in the column. U.S. Pat. No. 4,077,779, describes the use of a carbon molecular sieve that adsorbs CO2 selectively over hydrogen or methane. After the adsorption step, a high pressure purge with CO2 is followed by pressure reduction and desorption of CO2 followed by a rinse at an intermediate pressure with an extraneous gas such as air. The column is then subjected to vacuum to remove the extraneous gas and any remaining CO2. The preferred type of adsorbent is activated carbon, but can be a zeolite such as 5A, molecular sieve carbons, silica gel, activated alumina or other adsorbents selective of carbon dioxide and gaseous hydrocarbons other than methane.
U.S. Pat. No. 5,938,819 discloses removing CO2 from landfill gas, coal bed methane and coal mine gob gas, sewage gas or low quality natural gas in a modified PSA process using a clinoptilolite adsorbent. The adsorbent has such a strong attraction to CO2 that little desorption occurs even at very low pressure. There is such an extreme hysteresis between the adsorption of the adsorbent and desorption isotherms, regeneration of the adsorbent is achieved by exposure to a stream of dry air.
The adsorbent material selected for use in the pressure swing adsorption units depends on the component to be separated from the feed stream. Adsorbent materials suitable for use in the pressure swing adsorption apparatus include, but are by no means limited to, activated carbon; carbon molecular sieve (CMS) adsorbents; activated alumina; zeolites; and the titanium silicates, e.g., ETS and CTS materials described above. One skilled in the art would know how to select a given adsorbent material for use with a given feed gas mixture and desired product materials.
Carbon molecular sieves are well-known in the industry and are effective for separating oxygen from nitrogen because the rate of adsorption of oxygen is higher than that of nitrogen. A molecular sieve selectively adsorbs a certain size of molecules due to its uniform pore size, and a carbon molecular sieve is mainly made of a carbon material. The difference in rates of adsorption between oxygen and nitrogen is due to the difference in size of the oxygen and nitrogen molecules. Since the difference in size is quite small, approximately 0.2 A°, the pore structure of the carbon molecular sieve must be tightly controlled in order to effectively separate the two molecules. In order to improve the performance of carbon molecular sieves, various techniques have been used to modify pore size. A common method is the deposit of carbon on carbon molecular sieves. For example, U.S. Pat. No. 3,979,330 to Munzner et. al discloses the preparation of carbon containing molecular sieves in which coke containing up to 5% volatile components is treated at 600° C.-900° C. in order to split off carbon from a hydrocarbon. The split-off carbon is deposited in the carbon framework of the coke to narrow the existing pores. U.S. Pat. Nos. 4,528,281; 4,540,678; 4,627,857 and 4,629,476 to Jr. Robert, S. F. disclose various preparations of carbon molecular sieves for use in separation of gases.
U.S. Pat. No. 5,081,097 to Sharma et. al., discloses copper modified carbon molecular sieves for selective removal of oxygen from an argon gas mixture. The sieve is prepared by pyrolysis of a mixture of a copper-containing material and a polyfunctional alcohol to form a sorbent precursor. The sorbent precursor is then heated and reduced to produce a copper modified carbon molecular sieve. Pyrolysis is a high temperature process making the whole process of preparation of the adsorbent an energy intensive process.
Sometimes landfill gas upgrading requires the removal of nitrogen in addition to removing the oxygen. Where nitrogen is to be removed, the Molecular Gate® process (described earlier) has been applied downstream of a bulk membrane separation unit wherein residual CO2 and nitrogen is adsorbed using a PSA process. In the Molecular Gate® process, oxygen is also partially removed.